In general, a hydraulic excavator which is a typical example of a construction machine is composed of an automotive lower traveling structure and an upper revolving structure rotatably mounted on the lower traveling structure. On the front side of a revolving frame constituting the upper revolving structure, a working mechanism performing excavating work of earth and sand and the like is provided capable of moving upward/downward.
Here, the working mechanism of a hydraulic excavator is usually largely constituted by a boom having the base end side rotatably mounted on the revolving frame, an arm rotatably mounted on the distal end side of the boom, a working tool such as a bucket or the like rotatably mounted on the distal end side of the arm, and a boom cylinder, an arm cylinder, and a bucket cylinder driving the boom, the arm, and the bucket, respectively.
The arm constituting such working mechanism is usually formed as a lengthy welded structural body whose entire length is as long as several meters. That is, the arm is formed of left and right side plates, an upper plate joined to the upper end sides of these left and right side plates by welding, a lower plate joined to the lower end side of the left and right side plates by welding, and a rear plate joined to the rear end sides of the left and right side plates and the upper plate by welding. As a result, the arm is formed as a box-shaped structural body having a cross sectional surface of a square closed sectional structure.
On a rear part on the lower side of the arm, a boom connecting boss to be connected to the distal end side of the boom by using a connecting pin is provided, and on the rear end side of the arm, an arm cylinder bracket to which the arm cylinder is connected by using a connecting pin is provided. On the other hand, on the rear part on the upper side of the arm, a bucket cylinder bracket to which the bucket cylinder is connected by using a connecting pin is provided. Moreover, on the front end of the arm, a bucket connecting boss to which the bucket is connected by using a connecting pin is provided (Patent Document 1).